In a cathode ray tube (CRT) display, a phosphor screen is located opposite an electron gun. The electron beam emits electrons. The electrons are accelerated and focused on the phosphor screen by a high voltage grid. The phosphor screen is periodically refreshed.
An image is displayed on a phosphor screen. The phosphor screen is divided up into a number of horizontal scan lines. The electron beam is directed to the upper left corner of the phosphor screen at the first scan line when a new image is displayed. The electron beam is steered horizontally across each scan line at a fixed frequency. The electron beam returns to the left side of the phosphor screen after the electron beam reaches the right edge of the phosphor screen, a process called horizontal retrace.
During horizontal retrace, the electron beam is steered (right to left) to the left edge of the next scan line, which is immediately beneath the previous scan line. The beam is steered back to the top left corner of the phosphor screen during the vertical retrace interval after all of the scan-lines are traced by the electron beam. A horizontal deflection circuit steers the beam horizontally. A vertical deflection circuit steers the beam vertically. The horizontal and vertical deflection circuits produce high voltage signals that activate deflection coils.
Typical vertical deflection circuits include a vertical oscillator circuit and vertical deflection coils. A vertical pulse signal is coupled into the vertical deflection circuits. The vertical oscillator circuit is triggered by the sync pulse so that the vertical oscillator locks to the refresh frequency. The vertical oscillator generates a saw tooth waveform. The saw tooth waveform is used to generate a current ramp. The current ramp drives the vertical deflection coils such that the electron beam is steered from the top of the phosphor screen to the bottom of the phosphor screen at a uniform rate. At the end of the current ramp, the electron beam is steered to the top of the screen.
A distortion occurs in the image as the electron beam is steered from the image area at the side of the phosphor screen in a vertical direction. The top and bottom of the phosphor screen have a higher deflection angle with respect to the middle of the phosphor screen. Typically, an S-correction is performed on the image data to correct the resulting image distortion on the display. The deflection current is used to generate the vertical scan via the deflection coils. The deflection current is arranged as a saw tooth waveform. The slanted portions of the saw tooth waveform are modified by S-correction in an “S” shape. The top and bottom section of the sawtooth each have a small slope which results in a smaller deflection area.